Semiconductor device with self-aligned wavy contact profile and method of forming the same

ABSTRACT

A semiconductor device and method of manufacturing the semiconductor device are provided. An exemplary semiconductor device comprises a fin disposed over a substrate, wherein the fin includes a channel region and a source/drain region; a gate structure disposed over the substrate and over the channel region of the fin; a source/drain feature epitaxially grown in the source/drain region of the fin, wherein the source/drain feature includes a top epitaxial layer and a lower epitaxial layer formed below the top epitaxial layer, and the lower epitaxial layer includes a wavy top surface; and a contact having a wavy bottom surface matingly engaged with the wavy top surface of the lower epitaxial layer of the source/drain feature.

PRIORITY DATA

The present application is a continuation application of U.S. patentapplication Ser. No. 17/542,810, filed Dec. 6, 2021, which is adivisional application of U.S. patent application Ser. No. 16/656,619,filed Oct. 18, 2019, which claims benefit of U.S. Provisional PatentApplication No. 62/771,630, filed Nov. 27, 2018, each of which isincorporated herein by reference in its entirety.

BACKGROUND

The integrated circuit (IC) industry has experienced exponential growth.Technological advances in IC materials and design have producedgenerations of ICs, where each generation has smaller and more complexcircuits than the previous generation. In the course of IC evolution,functional density (i.e., the number of interconnected devices per chiparea) has generally increased while geometry size (i.e., the smallestcomponent (or line) that can be created using a fabrication process) hasdecreased. This scaling down process generally provides benefits byincreasing production efficiency and lowering associated costs.

Such scaling down has also increased the complexity of processing andmanufacturing ICs and, for these advances to be realized, similardevelopments in IC processing and manufacturing are needed. For example,as fin-like field effect transistor (FinFET) technologies progresstowards smaller feature sizes, various source/drain (S/D) materials, forexample, silicon germanium (SiGe), silicon phosphide (SiP) or siliconcarbide (SiC), have been explored to enhance carrier mobility forFinFETs. However, it has been observed that epitaxy source/drainfeatures may be damaged or loss during the S/D contact etching processand S/D contact area is limited in conventional FinFET fabrication.Accordingly, improvements are needed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 is a flowchart of an example method for making a semiconductordevice in accordance with some embodiments of the present disclosure.

FIG. 2 is a three-dimensional perspective view of an examplesemiconductor device in accordance with some embodiments of the presentdisclosure.

FIGS. 3A-16A are cross-sectional views of various embodiments of aportion of the semiconductor device in FIG. 2 , along the “A-A” line, inintermediate stages of fabrication according to an embodiment of themethod in FIG. 1 .

FIGS. 3B-16B are cross-sectional views of various embodiments of aportion of the semiconductor device in FIG. 2 , along the “B-B” line, inintermediate stages of fabrication according to an embodiment of themethod in FIG. 1 .

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact.

In addition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a feature on, connected to, and/or coupled toanother feature in the present disclosure that follows may includeembodiments in which the features are formed in direct contact, and mayalso include embodiments in which additional features may be formedinterposing the features, such that the features may not be in directcontact. In addition, spatially relative terms, for example, “lower,”“upper,” “horizontal,” “vertical,” “above,” “over,” “below,” “beneath,”“up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for easeof the present disclosure of one features relationship to anotherfeature. The spatially relative terms are intended to cover differentorientations of the device including the features. Still further, when anumber or a range of numbers is described with “about,” “approximate,”and the like, the term is intended to encompass numbers that are withina reasonable range including the number described, such as within +/−10%of the number described or other values as understood by person skilledin the art. For example, the term “about 5 nm” encompasses the dimensionrange from 4.5 nm to 5.5 nm.

The present disclosure is generally related to semiconductor devices andthe fabrication thereof, and more particularly to epitaxial S/D featuresfabrication of field-effect transistors (FETs), such as Fin-like FETs(FinFETs). According to some aspects of the present disclosure, theepitaxial S/D feature of the semiconductor device comprises a top layerof silicon arsenide (SiAs) or other pentavalent silicon compounddifferent than the material of other layers of the epitaxial S/D featureand can be etched anisotropically so that the later formed S/D contactmay form a self-aligned wavy contact profile (having at least two crestsand a valley between and below the two crests) to reduce thesource/drain resistance. With the protection of the different top layermaterial, the epitaxial S/D feature damage or loss may be mitigatedduring the S/D contact etching process, thus the performance of thesemiconductor device is improved.

FIG. 1 illustrates a flow chart of a method 100 for forming asemiconductor device (hereafter called “device” in short) in accordancewith some embodiments of the present disclosure. Method 100 is merely anexample and is not intended to limit the present disclosure beyond whatis explicitly recited in the claims. Additional operations can beperformed before, during, and after method 100, and some operationsdescribed can be replaced, eliminated, or moved around for additionalembodiments of the method. Method 100 is described below in conjunctionwith other figures, which illustrate various three-dimensional, planartop views, and cross-sectional views of an example device 200 duringintermediate steps of method 100. In particular, FIG. 2 illustrates athree-dimensional view of device 200. FIGS. 3A-16A illustratecross-sectional views of various embodiments of device 200 of FIG. 2taken along line A-A (along a Y-direction) at intermediate stages ofmethod 100 in accordance with some embodiments of the presentdisclosure. FIGS. 3B-16B illustrate cross-sectional views of variousembodiments of device 200 of FIG. 2 taken along line B-B (along anX-direction) at intermediate stages of method 100 in accordance withsome embodiments of the present disclosure.

Device 200 may be an intermediate device fabricated during processing ofan integrated circuit (IC), or a portion thereof, that may comprisestatic random-access memory (SRAM) and/or other logic circuits, passivecomponents such as resistors, capacitors, and inductors, and activecomponents such as p-type FETs (PFETs), n-type FETs (NFETs), fin-likeFETs (FinFETs), gate-all-around FETs (GAA FETs), metal-oxidesemiconductor field effect transistors (MOSFET), complementarymetal-oxide semiconductor (CMOS) transistors, bipolar transistors, highvoltage transistors, high frequency transistors, and/or other memorycells. Device 200 can be a portion of a core region (often referred toas a logic region), a memory region (such as a static random accessmemory (SRAM) region), an analog region, a peripheral region (oftenreferred to as an input/output (I/O) region), a dummy region, othersuitable region, or combinations thereof, of an integrated circuit (IC).In some embodiments, device 200 may be a portion of an IC chip, a systemon chip (SoC), or portion thereof. The present disclosure is not limitedto any particular number of devices or device regions, or to anyparticular device configurations. For example, though device 200 asillustrated is a three-dimensional FET device (e.g., a FinFET), thepresent disclosure may also provide embodiments for fabricating planarFET devices.

Referring to FIG. 2 , a semiconductor device 200 includes one or morefins 210 protruding from a substrate 202 and separated by an isolationstructure 204, and one or more gate stacks 260 disposed over substrate202 and fins 210. Gate stacks 260 defines a channel region (covered bythe gate stacks 260), a source region and a drain region of fins 210.Gate spacers 230 disposed along sidewalls of gate stacks 260. Gatestacks 260 may include components such as one or more gate dielectriclayers 262 disposed over isolation structure 204 and substrate 202, abarrier layers (not shown), a glue layer (not shown), a gate electrode264 disposed over gate dielectric layers 262, and one or more hard masklayers 266 disposed over gate electrode 264, other suitable layers, orcombinations thereof. Device 200 also include source/drain features 250epitaxially grown over source/drain (S/D) regions of fins 210. Device200 may also comprise an interlayer dielectric (ILD) layer 270 depositedover substrate 202, isolation structure 204, fins 210, and source/drainstructures 250. Source/drain contacts 280 are formed over epitaxial S/Dfeatures 250 in the source/drain region of fins 210. Forming of device200 is discussed in method 100 of FIG. 1 along with different views ofdevice 200 at intermediate stages of method 100 as illustrated in FIGS.3A-15A and 3B-15B.

Referring to FIGS. 1 and 3A-3B, at operation 105, a substrate 202 isprovided and one or more fins 210 are formed over substrate 202. Lowerportions of fins 210 are separated by an isolation structure 204.

In the depicted embodiment of FIG. 3A-3B, device 200 comprises asubstrate (wafer) 202. In the depicted embodiment, substrate 202 is abulk substrate that includes silicon. Alternatively or additionally, thebulk substrate includes another elementary semiconductor, a compoundsemiconductor, an alloy semiconductor, or combinations thereof.Alternatively, substrate 202 is a semiconductor-on-insulator substrate,such as a silicon-on-insulator (SOI) substrate, a silicongermanium-on-insulator (SGOI) substrate, or a germanium-on-insulator(GOI) substrate. Semiconductor-on-insulator substrates can be fabricatedusing separation by implantation of oxygen (SIMOX), wafer bonding,and/or other suitable methods. Substrate 202 may include various dopedregions. In some embodiments, substrate 202 includes n-type dopedregions (for example, n-type wells) doped with n-type dopants, such asphosphorus (for example, ³¹P), arsenic, other n-type dopant, orcombinations thereof. In some embodiments, substrate 202 includes p-typedoped region (for example, p-type wells) doped with p-type dopants, suchas boron (for example, ¹¹B, BF2), indium, other p-type dopant, orcombinations thereof. An ion implantation process, a diffusion process,and/or other suitable doping process can be performed to form thevarious doped regions.

Semiconductor fins 210 are formed over substrate 202. Each fin 210 maybe suitable for providing an n-type FET or a p-type FET. Fins 210 areoriented substantially parallel to one another. Each of fins 210 has atleast one channel region 210C and at least one source region and drainregion (both refer to source/drain region 2105/D) defined along theirlength in the X-direction, where the at least one channel region 210C iscovered by gate structures and is disposed between the source/drainregions 2105/D. In some embodiments, fins 210 are portions of substrate202 (such as a portion of a material layer of substrate 202). Forexample, in the depicted embodiment, where substrate 202 includessilicon, fins 210 include silicon. Alternatively, in some embodiments,fins 210 are defined in a material layer, such as one or moresemiconductor material layers, overlying substrate 202. For example,fins 210 can include a semiconductor layer stack having varioussemiconductor layers (such as a heterostructure) disposed over substrate202. The semiconductor layers can include any suitable semiconductormaterials, such as silicon, germanium, silicon germanium, other suitablesemiconductor materials, or combinations thereof. The semiconductorlayers can include same or different materials, etching rates,constituent atomic percentages, constituent weight percentages,thicknesses, and/or configurations depending on the design of device200. Fins 210 are formed by any suitable process including variousdeposition, photolithography, and/or etching processes.

Isolation structure 204 is formed over substrate 202 and separates thelower portions of fins 210. Isolation structure 204 electricallyisolates active device regions and/or passive device regions of device200. Isolation structure 204 can be configured as different structures,such as a shallow trench isolation (STI) structure, a deep trenchisolation (DTI) structure, a local oxidation of silicon (LOCOS)structure, or combinations thereof. Isolation structure 204 includes anisolation material, such as silicon oxide, silicon nitride, siliconoxynitride, other suitable isolation material, or combinations thereof.Isolation structure 204 is deposited by CVD, physical vapor deposition(PVD), atomic layer deposition (ALD), high density plasma CVD (HDPCVD),metal organic CVD (MOCVD), remote plasma CVD (RPCVD), PECVD, LPCVD,atomic layer CVD (ALCVD), atmospheric pressure CVD (APCVD), othersuitable deposition process, or combinations thereof. In someembodiments, isolation structure 204 is formed before fins 210 areformed (an isolation-first scheme). In some other embodiments, fins 210are formed before isolation structure 204 is formed (a fin-firstscheme). A planarization process, such as a chemical mechanicalpolishing (CMP) process, can be performed on isolation structure 204.

In the depicted embodiment of 3A, each fin 210 has a height FH along theZ-direction. The fin height FH is a height between the top surface ofSTI structure 204 and the top surface of fins 210. In some embodiments,the height FH of each fin 210 is between about 50 nanometers (nm) andabout 80 nm. Also, in the embodiments of FIG. 3A, each fin 210 has arelatively tapered shape, as shown. In other embodiments, the fins canbe less tapered (or not tapered), or the taper can be multi-faceted. Forexample, a portion of the fin above the STI structure 204 can be at agreater taper angle than a portion of the fin directly surrounded by theSTI structure.

Referring to FIGS. 1 and 4A-4B, at operation 110, various dummy gatestructures are formed over channel region 210C of fins 210. “Dummy gatestructure” generally refers to an electrically non-functional gatestructure of device 200. In some embodiments, a dummy gate structuremimics physical properties of an active gate structure, such as physicaldimensions of the active gate structure, yet is relativelynon-conductive (in other words, does not enable current to flow). Dummygate structures extend along Y-direction and traverse respective fin210. Each dummy gate structure may include a dummy gate stack 220 andspacers 230 deposited along sidewalls of dummy gate stack 220. In someembodiments, each dummy gate stack 220 includes, for example, a dummygate dielectric layer 222 (including, for example, silicon oxide) and adummy gate electrode 224 (including, for example, polysilicon). Dummygate dielectric layer 222 includes a dielectric material, such assilicon oxide, a high-k dielectric material, other suitable dielectricmaterial, or combinations thereof. High-k dielectric material generallyrefers to dielectric materials having a high dielectric constant, forexample, greater than that of silicon oxide (k≈3.9). Dummy gate stacks220 can include numerous other layers, for example, capping layers,interface layers, diffusion layers, barrier layers, hard mask layers, orcombinations thereof. Dummy gate stacks 220 are formed by depositionprocesses, lithography processes, etching processes, other suitableprocesses, or combinations thereof.

Each dummy gate structure may include spacers 230 formed along sidewallsof dummy gate stack 220. In some embodiments, spacers 230 comprise asingle layer dielectric material. In some other embodiments, spacers 230may comprise a multi-layer structure comprising different dielectricmaterials with different etching selectivity. The dielectric materialcan include silicon, oxygen, carbon, nitrogen, other suitable material,or combinations thereof (for example, silicon oxide, silicon nitride,silicon oxynitride, silicon carbide, or silicon carbonitride). Spacers230 are formed by any suitable process. For example, in the depictedembodiment, a spacer layer may be deposited conformally over substrate202, isolation structure 204, dummy gate stacks 220 and fins 210.Subsequently, the spacer layer is anisotropically etched to form spacers230 along dummy gate stacks 220.

Referring to FIGS. 1 and 5A-5B, at operation 115, source/drain features250 are epitaxially grown in the source/drain region 2105/D of fins 210.In some embodiments, semiconductor material is epitaxially grown on fins210, forming epitaxial S/D features 250. In some embodiments, a finrecess process (for example, an etch back process) is performed onsource/drain regions 2105/D of fins 210, such that epitaxial S/Dfeatures 250 are grown from lower fin active regions. In some otherembodiments, source/drain regions 2105/D of fins 210 are not subjectedto a fin recess process, such that epitaxial S/D features 250 are grownfrom and wrap at least a portion of upper fin active regions.

In some embodiments, epitaxial S/D features 250 comprises one or moreepitaxial layers grown on S/D regions 2105/D of fins 210. For example, afirst epitaxial layer is deposited on the top and sidewall surfaces ofthe S/D regions of fins 210. In other words, the first epitaxial layerwraps around S/D regions 210S/D of fins 210. Further, a second epitaxiallayer wraps around the first epitaxial layer. In some furtherembodiments, a third and/or a fourth epitaxial layer may further wraparound the prior epitaxial layer.

Depending on the lateral distance (along the Y-direction) between twoadjacent fins 210 and the control of the epitaxial growth, epitaxial S/Dfeatures 250 may be formed to have different merging profiles. In someembodiments, epitaxial S/D features 250 are grown around each fin 210,separately. That is, each epitaxial S/D features 250 is separated andnone of the epitaxial layers is merged (i.e., touch each other). In someother embodiments, as depicted in FIG. 5A, epitaxial S/D features 250forming around fins 210 are laterally merged together along theY-direction and span more than one fin 210. In a further embodiment, theinner epitaxial layer (for example, the first epitaxial layer of atwo-layer epitaxial S/D feature) on fins 210 do not merge, but the outerepitaxial layer (for example, the second epitaxial layer of a two-layerepitaxial S/D feature) on fins 210 may laterally merge together. Inanother further embodiment, both the inner epitaxial layer(s) and theouter epitaxial layer(s) on the two adjacent fins 210 laterally merge.In some embodiments, to form the structure that both the inner and outerepitaxial layers laterally merge, the operation 115 waits until theinner epitaxial layer(s) laterally merge before growing the outerepitaxial layer(s).

Epitaxial S/D features 250 may comprise semiconductor materials such assilicon (Si), phosphorus (P), silicon phosphide (SiP), silicon carbide(SiC), germanium (Ge), silicon germanium (SiGe), one or more III-Vmaterials, a compound semiconductor, or an alloy semiconductor. In someembodiments, epitaxial S/D features 250 are doped with n-type dopantsand/or p-type dopants. For example, in an n-type FinFET region,epitaxial S/D features 250 may include epitaxial layers includingsilicon and/or carbon, where silicon-containing epitaxial layers orsilicon-carbon-containing epitaxial layers are doped with phosphorous,arsenic, other n-type dopant, or combinations thereof (for example,forming an Si:P epitaxial layer, an Si:C epitaxial layer, or an Si:C:Pepitaxial layer). In furtherance of the example, in a p-type FinFETregion, epitaxial S/D features 250 may include epitaxial layersincluding silicon and/or germanium, where the silicon germaniumcontaining epitaxial layers are doped with boron, carbon, other p-typedopant, or combinations thereof (for example, forming an Si:Ge:Bepitaxial layer or an Si:Ge:C epitaxial layer). In some embodiments,epitaxial S/D features 250 include materials and/or dopants that achievedesired tensile stress and/or compressive stress in the channel regions.In various embodiments, different epitaxial layer(s) of epitaxial S/Dfeatures 250 may comprise same or different semiconductor materials.Different embodiments of epitaxial S/D features 250 will be discussed inFIGS. 6A-6B, 11A-11B, 13A-13B, and 15A-15B.

An epitaxy process can implement CVD deposition techniques (for example,vapor-phase epitaxy (VPE), ultra-high vacuum CVD (UHV-CVD), LPCVD,and/or PECVD), molecular beam epitaxy, other suitable SEG processes, orcombinations thereof. In some implementations, epitaxial S/D features250 are doped during deposition by adding impurities to a sourcematerial of the epitaxy process. In some implementations, epitaxial S/Dfeatures 250 are doped by an ion implantation process subsequent to adeposition process. In some implementations, annealing processes areperformed to activate dopants in epitaxial S/D features 250 and/or othersource/drain features of device 200.

As shown in FIG. 5A, the epitaxial S/D features 250 can have arelatively-diamond shaped cross-section, with a significant portionexisting above the fin 210. In other embodiments, the features can bemore rounded and curved in shape, and/or may have a relatively smallportion (or no portion) existing above the fin.

Referring to FIGS. 6A and 6B, at operation 115, in an embodiment of ann-type FinFET region, a merged epitaxial S/D feature 250-1 wrappingaround two adjacent fins 210 is formed. In other words, epitaxial S/Dfeature 250-1 is a merged S/D feature across two adjacent fins 210. Inthe depicted embodiment, fins 210 comprise Si, and epitaxial S/Dfeatures 250-1 is a three-layer epitaxial S/D feature comprising threedifferent epitaxial layers L1, L2, and L3. In the depicted embodiment,epitaxial S/D feature 250-1 comprises a first epitaxial layer L1comprising silicon phosphide (SiP). In some other embodiments, firstepitaxial layer L1 may comprise Si or SiC. In the depicted embodiment,each first epitaxial layer L1 respectively wraps an upper active regionof each fin 210. The first epitaxial layers L1 do not merge or acrossadjacent fins. Epitaxial S/D features 250-1 further comprises a secondepitaxial layer L2 comprising SiP and wrapping around the firstepitaxial layer L1. In this embodiment, the material SiP in the secondepitaxial layer L2 has a different etching selectivity than the SiPincluded in the first epitaxial layer L1. For example, the firstepitaxial layer L1 comprises SiP having a P doping concentration ofabout 5×10²⁰ to about 5×10²¹ atoms/cm³ (a molar ratio of P is less thanabout 2%); and the second epitaxial layer L2 comprises SiP having a Pdoping concentration of about 5×10²¹ to about 5×10²² atoms/cm³ (a molarratio of P is about 2% to about 10%). In the depicted embodiment, thesecond epitaxial layer L2 is a merged epitaxial layer with across-section shape as two merged rounded diamond shapes. The secondepitaxial layer L2 has a wavy top surface and a wavy bottom surface asdepicted in FIG. 6A, wherein the wavy top surface has at least twocrests and a valley between and below the two crests, and the wavybottom surface has two valleys and a crest between and above the twovalleys.

Epitaxial S/D feature 250-1 further comprises a third epitaxial layerL3, which is the top epitaxial layer comprising silicon arsenide (SiAs).In some embodiments, the third (top) epitaxial layer L3 comprises SiAshaving an As doping concentration of about 1×10²² to about 1×10²³atoms/cm³. In some embodiments, a molar ratio of As in the thirdepitaxial layer L3 is about 2% to about 10%. In some other embodiments,another pentavalent may be used to form the silicon compound in thethird epitaxial layer L3 (such as antimony (Sb), Bismuth (Bi), moscovium(Mc), other pentavalent silicon compound, or combinations thereof) sothat the semiconductor material of top epitaxial layer L3 is differentfrom the semiconductor material of the second epitaxial layers L2 andhas an etching selectivity different from that of the semiconductormaterial of the second epitaxial layer L2. As depicted in FIG. 6A, thethird epitaxial layer L3 is directly formed over the second epitaxiallayer L2 and the bottom surface of the third epitaxial layer L3 matches(e.g., is coplanar with) the top surface of the second epitaxial layerL2, both have the same wave profile. As depicted in FIG. 6B, the thirdepitaxial layer L3 contacts both the first epitaxial layer L1 and thesecond epitaxial layer L2 in the X-direction.

Epitaxial S/D features 250-1 is formed by an epitaxial growth process.The epitaxial growth process may be a LPCVD process with a silicon-basedprecursor, a selective epitaxial growth (SEG) process, a cyclicdeposition and etching (CDE) process, other suitable processes, orcombinations thereof. For example, silicon phosphide crystal may beformed with a chemical gas (Si₃H₈, SiH₃CH₃ and/or PH₃) based depositionprocess combined with a selective chemical vapor etch process. Foranother example, silicon crystal may be grown with LPCVD withdichlorosilane (SiH₂Cl₂) as the precursor. The precursor may be dopedin-situ (during the epitaxial growth process) or ex-situ (after theepitaxial growth process is completed) with n-type dopants, for examplephosphorus (P) for L1 and L2 and arsenic (As) for L3, respectively, toform the different epitaxial layers of epitaxial S/D feature 250-1. Insome embodiments, different epitaxial layers of epitaxial S/D feature250-1 are doped during deposition by adding impurities to a sourcematerial of the epitaxy process. In some embodiments, different layersof epitaxial S/D feature 250-1 are doped by an ion implantation processsubsequent to a deposition process. Implant energies and dosages duringthe ion implantation process can be configured depends on the design ofdevice 200. For example, first epitaxial layer L1 is formed by ionimplantation doping with a lower concentration of P (for example, amolar ratio of P is less than about 2%) to the silicon precursor for athickness T1 of about 1 nm to about 10 nm; second epitaxial layer L2 isformed by ion implantation doping with a higher concentration of P (forexample, a molar ratio of P is about 2% to about 10%) to the siliconprecursor for a thickness T2 of about 10 nm to about 40 nm; and thirdepitaxial layer L3 is formed by ion implantation doping with As (forexample, about 1×10²² to about 1×10²³ atoms/cm³ of As, in someembodiments, about 2% to about 10% of As) to the silicon precursor for athickness T3 of about 0.1 nm to about 20 nm. In some embodiments, athickness ratio of the top (third) epitaxial layer L3 to the bottom(first) epitaxial layer L1 is about 0.01 to about 20; a thickness ratioof the top (third) epitaxial layer L3 to the second epitaxial layer L2is about 0.0025 to about 2. In some embodiment, a ratio of the thicknessof the top (third) epitaxial layer L3 comprising SiAs to the height ofepitaxial S/D features 250-1 (T1+T2+T3) is about 1% to about 20%. Asdepicted in FIGS. 6A and 6B, epitaxial S/D features 250-1 has a mergewidth W1 in the Y-direction and a width W2 in the X-direction. In someembodiments, the merge width W1 is about 5 nm to about 100 nm, and thewidth W2 is about 5 nm to about 25 nm. In some embodiments, a ratio ofthe thickness T3 of the top epitaxial layer L3 to a merge width W1 ofepitaxial S/D features 250-1 is about 0.001 to about 4, and a ratio ofthe thickness T3 of the top epitaxial layer L3 to a width W2 ofepitaxial S/D features 250-1 is about 0.004 to about 4. In someembodiments, annealing processes (such as rapidthermal/milli-second/laser anneal) are performed to activate dopants inepitaxial S/D features 250-1 and/or other source/drain features ofdevice 200, such as HDD regions and/or LDD regions.

Using SiAs as the top epitaxial layer to form epitaxial S/D features forn-type FET devices provides several benefits. For example, using SiAs inthe top layer (different than material of other layers) ensures a betteretching selectivity than using SiP in the top layer (same/similar asmaterial of other layers) in epitaxial S/D feature of an n-type FETdevice. Therefore, in a later S/D contact selective etching process, theanisotropic etching can selectively etch the wavy surface of theepitaxial S/D feature (for example, the top surface of second epitaxiallayer L2) such that the S/D contact may have an enlarged wavy contactsurface with the epitaxial S/D feature, thereby reducing the contactresistance between the S/D contact and the epitaxial S/D feature.Moreover, the selectivity between SiAs and SiP is relatively strong,which can mitigate any epitaxial S/D feature (for example, secondepitaxial layer L2) damage or loss during the S/D contact selectiveetching process. Furthermore, dopant activation within SiAs may beachieved using a lower thermal budget (e.g., lower activation annealtemperature and/or time), for example, as compared to dopant activationwithin SiP. In some embodiments, the thermal budget used to achievedopant activation in SiAs may be about 15-20% lower than that used toactivate dopants in SiP. Additionally, in some embodiments, a dopedlayer formed using SiAs may be thinner than a doped layer formed usingSiP. For example, a doped layer formed using SiAs may be about 0.5-0.8times the thickness of a doped layer formed using SiP.

In an alternative fabrication process, the top epitaxial layer of theepitaxial S/D feature of an n-type device may be a layer comprises SiP,same or similar (only different in P doping concentration or molarratio) as other layers of the epitaxial S/D feature. Even though themolar ratio or the doping concentration of SiP in the top epitaxiallayer (for example, the third epitaxial layer L3 may have a P dopingconcentration of about 1×10²¹ to about 5×10²¹ atoms/cm³) is differentthan that in the next lower epitaxial layer (for example, the secondepitaxial layer L2 may having a P doping concentration of about 5×10²⁰to about 5×10²¹ atoms/cm³), etching selectivity between the topepitaxial layer and the next lower epitaxial layer may be relativelyweak. Therefore, in the later metal contact selective etching process,residual portions of the top epitaxial layer in the valley (below thecrest) of the wavy surface of the next lower epitaxial layer may exist.Accordingly, the contact surface between the S/D contact and theepitaxial S/D feature may not be a wavy surface, but instead will berelatively flat and small, as compared to the wavy contact surfaceformed by using SiAs as the top epitaxial layer. In addition, due to thepoor selectivity between the top epitaxial layer and the next lowerepitaxial layer (both using SiP, although with different molar ratios),portions of the next lower epitaxial layer may be removed when S/Dcontact etching is performed to remove the top epitaxial layer. Thus,using of SiAs as the top epitaxial layer of epitaxial S/D featuresprovides many benefits and can improve the performance of thesemiconductor device.

Now referring to FIGS. 1 and 7A-7B, at operation 120, a metal gatereplacement process is performed to replace dummy gate stack 220 with ametal gate stack 260. First, an interlayer dielectric (ILD) layer 270 isformed over substrate 202, particularly over source/drain structures250, dummy gate structures 220, and fins 210. ILD layer 270 includes adielectric material including, for example, silicon oxide, siliconnitride, silicon oxynitride, TEOS formed oxide, PSG, BPSG, low-kdielectric material, other suitable dielectric material, or combinationsthereof. In some embodiments, ILD layer 270 has a multilayer structurehaving multiple dielectric materials. In some embodiments, a etch stoplayer (ESL) 268 is disposed between ILD layer 270 and source/drainstructures 250, dummy gate structures 220, and/or fins 210. ESL 268includes a dielectric material different than ILD layer 270. ILD layer270 and/or ESL are formed, for example, by a deposition process (such asCVD, FCVD, PVD, ALD, HDPCVD, MOCVD, RPCVD, PECVD, LPCVD, ALCVD, APCVD,plating, other suitable methods, or combinations thereof). Subsequent tothe deposition of ILD layer 270 and/or ESL 268, a CMP process and/orother planarization process is performed until reaching (exposing) topsurfaces of dummy gate structures 220.

Subsequently, dummy gate stacks 220 are removed to exposes channelregions 210C of fins 210. The removing process may be an etchingprocess, which may include a dry etching process, a wet etching process,or combinations thereof. Thereafter, metal gate stacks 260 are formedover channel regions 210C of fins 210. Metal gate stacks 260 are formedby a proper procedure, such as a gate-last process or a high-k-lastprocess. Each of metal gate stacks 260 may include a gate dielectriclayer 262, a gate electrode 264 disposed over gate dielectric layer 262,and a hard mask layer 266 disposed over gate electrode 264. Referring toFIG. 7B, gate dielectric layer 262 is deposited over channel regions210C of fins 210. Gate dielectric layer 262 include a dielectricmaterial, such as silicon oxide, high-k dielectric material, othersuitable dielectric material, or combinations thereof. In someembodiments, the high-k dielectric material has a dielectric constantgreater than or equal to about five (k≥5). Gate dielectric layer 262 isformed by various processes, such as ALD, CVD, PVD, and/or othersuitable process. Gate electrode 264 may comprise a work function layerand a metal fill layer. The work function layer may include a conductivelayer of metal or metal alloy with proper work function such that thecorresponding FET is enhanced for its device performance (for example,reduce the threshold voltage of the semiconductor device). The workfunction layer is different in composition for a p-FET and an n-FET,respectively referred to as an p-type work function (WF) metal and ann-type WF metal. The work function metal may be deposited by a suitableprocess, such as ALD, CVD, PVD, and/or other suitable process, over gatedielectric layer 262. The metal fill layer of gate electrode 264 mayinclude metal such as aluminum, tungsten, copper or other suitablemetal, and is disposed over work function layer by a suitable technique,such as PVD or plating. A hard mask layer 266 including, for example,silicon nitride or silicon carbide, is then disposed over gate electrode264 by a deposition processes, such as ALD, CVD, PVD, and/or othersuitable process. A CMP process can be performed to remove any excessmaterial of metal gate stacks 260 and planarizing a top surface thereof.

Now referring to FIGS. 1 and 8A-8B, at operation 125, source/drain (S/D)contacts 280 are formed over source/drain regions 2105/D of fins 210. Insome embodiment, a photoresist mask 275 (illustrated in dashed line) ispatterned to expose a landing window over source/drain regions 2105/D offins 210 to form S/D contacts 280. Patterning mask 275 may includeexposing a mask layer (for example, comprising silicon oxide, siliconnitride, or silicon oxynitride) to a pattern through a process such asphotolithography, performing a post-exposure bake process, anddeveloping the photoresist mask. Patterning may also be implemented orreplaced by other proper methods, such as maskless photolithography,electron-beam writing, ion-beam writing, and molecular imprint.

Portions of ILD layer 270 and the top epitaxial layer of epitaxial S/Dfeatures 250 exposed through photoresist mask 275 are subsequentlyremoved to form a landing window (a trench) for S/D contact 280. Theremoving process may include a plasma etch, a reaction ion etch (RIE), adry etch, a wet etch, another proper removing process, or combinationsthereof. For example, referring to FIG. 8A and 8B where the topepitaxial layer L3 comprises SiAs, an anisotropical selective etch usinga chlorine (Cl)-based plasma chemistry is performed. In someembodiments, the Cl-based anisotropical etch is a two-step etchingprocess. For example, in a first step, boron trichloride (BCl₃, forexample, about 10-20 sccm) with a certain power density (for example,about 0.1-0.5 W/cm²) and pressure (for example, about 15-30 mTorr) isused to remove portions of ILD layer 270 (for example, comprisingsilicon oxide) in the landing window of S/D contact; in a second step, agas mixture of Cl₂ (for example, about 15-25 sccm) and BCl₃ (forexample, about 1-10 sccm) with a certain power density (for example,about 0.1-1 W/cm²) and pressure (for example, about 10-20 mTorr) is usedto remove portions the top epitaxial layer L3 (comprising SiAs) ofepitaxial S/D features 250. In principle, undoped or lightly dopedsilicon is not etched by Cl atoms or Cl2 molecules unless energetic ionbombardment in Cl-containing plasma is present. Thus, the Cl basedplasma etch achieves a vertical (anisotropical) removal of the topepitaxial layer L3. In addition, Cl atoms are covalently bound tospecific sites in the case of an undoped silicon surface, which preventsor reduces the etching of the silicon. However, in a doped (e.g., witharsenic) silicon surface, a more ionic Si—Cl bound is formed on thesilicon surface which enhances electron transfer and promotes thesilicon etching. Therefore, as depicted in FIG. 9A and 9B, top epitaxiallayer L3 (comprising SiAs) within the landing window (trench) issubstantially removed, thus the wavy surface (having at least two crestsand a valley between and below the two crests as depicted in FIG. 9A) ofthe second epitaxial layer L2 (comprising SiP) is exposed. In addition,because of the relatively strong selectivity between SiAs (top epitaxiallayer) and SiP (second epitaxial layer), the second epitaxial layer L2is substantially unchanged during the selective Cl-based plasma etching.

Thereafter, referring to FIGS. 8A, 8B, 9A, and 9B, S/D contacts 280 areformed within the landing window (trench) and over the wavy top surfaceof second epitaxial layer L2. S/D contacts 280 may comprise metals suchas copper, ruthenium or cobalt. Various deposition process may beapplied to deposit S/D contacts 280. For example, the deposition of thecopper may include PVD to form a seed layer and plating to form bulkcopper on the copper seed layer. In some embodiments, prior to fillingconductive material in contact holes, silicide may be formed on thesource/drain regions to further reduce the contact resistance. In someembodiments, the silicide is considered as a portion of S/D contact 280.The silicide includes silicon and metal, such as titanium silicide,tantalum silicide, nickel silicide or cobalt silicide. The silicide maybe formed by a process referred to as self-aligned silicide (orsalicide). The process includes metal deposition, annealing to react themetal with silicon, and etching to remove unreacted metal. As depictedin FIG. 9A, S/D contact 280 has sidewalls partially contacts sidewallsof top (third) epitaxial layer L3 and S/D contact 280 has a wavy bottomsurface matches and directly contacts (matingly engaged with) the wavytop surface of second epitaxial layer L2 within the landing window(trench) formed in the top epitaxial layer L3. As depicted in FIG. 9B,S/D contact 280 contacts both the first epitaxial layer L1 and thesecond epitaxial layer L2 in the X-direction. In the depictedembodiment, the top (third) epitaxial layer L3 surrounds a lower portionof S/D contact 280. The top (third) epitaxial layer L3 includes threesurfaces: surface L3S1 contacting the lower portion of the sidewall ofS/D contact 280; surface L3S2 contacting a portion of the secondepitaxial layer L2; and surface L3S3 contacting ILD 270. As depicted inFIG. 9A, the bottom surface of S/D contact 280 has substantially thesame wavy profile as at least a portion of the top surface of the secondepitaxial layer L2. The wavy contact surface has at least two crests anda valley between and below the two crests. The crests and the valley ofthe wavy contact surface enlarges the contact surface between S/Dcontact 280 and epitaxial feature 250-1 compare to the conventional flatcontact surface between the S/D contact and the epitaxial S/D feature.Therefore, the contact resistance between the S/D contact and theepitaxial S/D feature is reduced and the performance of device 200 isimproved.

Referring to FIGS. 1, 10A and 10B, at operation 130, further processingis performed to complete the fabrication of device 200. For example,various other contacts, vias (such as vias 290), metal wires, dielectriclayers (such as ESL 282 and ILD 284) in a multilayer interconnect (MLI)feature are formed over substrate 202, so that the MLI feature canconnect various features to form a functional circuit that may includeone or more FET devices.

Using SiAs or other pentavalent silicon compound as the top epitaxiallayer of epitaxial S/D features not only benefits three-layer epitaxialS/D features in a n-type device, but also other number of layers (forexample, two layers or four layers) of epitaxial S/D features and/or ina p-type device. FIGS. 11A-16A and 11B-16B provide various embodimentsof epitaxial S/D features and the S/D contacts having wavy profiles indevice 200. FIGS. 11A-16A are cross-sectional views of a portion ofdevice 200 along the “A-A” line, in accordance with some embodiments ofthe present disclosure. FIGS. 11B-16B are cross-sectional views of aportion of device 200 along the “B-B” line, in accordance with someembodiments of the present disclosure.

FIGS. 11A and 11B provides a device 200 having a four-layer epitaxialS/D feature 250-2 in an n-type region. Epitaxial S/D feature 250-2comprises a first epitaxial layer L1, a second epitaxial layer L2, athird epitaxial layer L3, and a fourth epitaxial layer L4. In thedepicted embodiment, each first epitaxial layer L1 respectively wraps anupper active region of each fin 210. The first epitaxial layers L1 donot merge or across adjacent fins. The second epitaxial layer L2 is amerged epitaxial layer and is wrapped by the first epitaxial layer L1 inthe X-direction; the third epitaxial layer L3 is formed over the secondepitaxial layer L2 and having a wavy bottom surface directly contact awavy top surface of the second epitaxial layer L2; and the fourthepitaxial layer L4 is formed over the third epitaxial layer L3 andhaving a wavy bottom surface directly contact a wavy top surface of thethird epitaxial layer L3. In the embodiment, epitaxial layers L1-L3 allcomprise SiP, but with different molar ratios of P. For example, a molarratio of P in the first epitaxial layer L1 is less than about 2%; amolar ratio of P in the second epitaxial layer L2 is about 2% to about10%; and a molar ratio of P in the third epitaxial layer L3 is more thanabout 10%. In the depicted embodiment, the fourth epitaxial layer L4 isthe top epitaxial layer comprising SiAs, wherein, for example, a molarratio of As in the fourth epitaxial layer L4 is about 2% to about 10%.In some embodiments, a doping concentration of As in fourth epitaxiallayer L4 is about 1×10²² to about 1×10²³ atoms/cm³. The forming processof epitaxial S/D feature 250-2 is similar to that of the epitaxial S/Dfeature 250-1 as previously discussed. In some embodiments, a thicknessT1 of the first epitaxial layer L1 is about 1 nm to about 10 nm; athickness T2 of the second epitaxial layer L2 is about 10 nm to about 40nm; a thickness T3 of the third epitaxial layer L3 is about 10 nm toabout 40 nm; and a thickness T4 of the fourth epitaxial layer L4 isabout 0.1 nm to about 20 nm. Thus, a thickness ratio of the top (fourth)epitaxial layer L4 to the bottom (first) epitaxial layer L1 is about0.01 to about 20; a thickness ratio of the top (fourth) epitaxial layerL4 to the second epitaxial layer L2 is about 0.0025 to about 2; and athickness ratio of the top (fourth) epitaxial layer L4 to the thirdepitaxial layer L3 is about 0.0025 to about 2. In some embodiment, aratio of the thickness of the top (fourth) epitaxial layer L4 comprisingSiAs to the height of epitaxial S/D features 250-2 (T1+T2+T3+T4) isabout 1% to about 20%. As depicted in FIGS. 11A and 11B, epitaxial S/Dfeatures 250-2 has a merge width W1 in the Y-direction and a width W2 inthe X-direction. In some embodiments, the merge width W1 is about 5 nmto about 100 nm, and the width W2 is about 5 nm to about 25 nm. In someembodiments, a ratio of the thickness T4 of the top epitaxial layer L4to the merge width W1 of epitaxial S/D features 250-2 is about 0.001 toabout 4, and a ratio of the thickness T4 of the top epitaxial layer L4to a width W2 of epitaxial S/D features 250-2 is about 0.004 to about 4.

Referring to FIGS. 12A and 12B, portions of top epitaxial layer L4comprising SiAs is removed by a process, for example, an anisotropicalchlorine-based plasma etching process as previously discussed.Therefore, a trench is formed in the top epitaxial layer L4. The thirdepitaxial layer L3 comprising SiP is substantially unchanged due to therelatively strong selectivity between SiAs (comprised in top epitaxiallayer) and SiP. A wavy top surface (having at least two crests and avalley between and below the two crests as shown in FIG. 12A) of thethird epitaxial layer L3 is exposed from the trench and a source/draincontact 280 is then directly formed over the exposed wavy top surface ofthe third epitaxial layer L3. As depicted in FIG. 12A, S/D contact 280has a wavy bottom surface matching (matingly engaged with) the wavy topsurface of third epitaxial layer L3 within the landing window (trench)formed in the top epitaxial layer L4. As depicted in FIG. 12B, in theX-direction, S/D contact 280 contacts the third epitaxial layer L3 whichcontacts both the first epitaxial layer L1 and the second epitaxiallayer L2. In the depicted embodiment, the top (fourth) epitaxial layerL4 of epitaxial feature 250-2 surrounds a lower portion of S/D contact280. The top (fourth) epitaxial layer L4 includes three surfaces:surface L4S1 contacting the lower portion of the sidewall of S/D contact280; surface L4S2 contacting a portion of the third epitaxial layer L3;and surface L4S3 contacting ILD 270. As depicted in FIG. 12A, the bottomsurface of S/D contact 280 has substantially the same wavy profile as atleast a portion of the top surface of the third epitaxial layer L3. Thewavy contact surface has at least two crests and a valley between andbelow the two crests. The crests and the valley of the wavy contactsurface enlarges the contact surface between S/D contact 280 andepitaxial S/D feature 250-2 compare to the conventional flat contactsurface between the S/D contact and the epitaxial S/D feature.Therefore, the contact resistance between the S/D contact and theepitaxial S/D feature is reduced and the performance of device 200 isimproved.

FIGS. 13A and 13B provides a device 200 having a three-layer epitaxialS/D feature 250-3 in a p-type region. Epitaxial S/D feature 250-3includes a first epitaxial layer L1, a second epitaxial layer L2, and athird epitaxial layer L3. In the depicted embodiment, epitaxial layersL1 and L2 both comprise silicon germanium (SiGe), but with differentmolar ratios of germanium (Ge). For example, a molar ratio of Ge in thefirst epitaxial layer L1 is less than about 40%; and a molar ratio of Gein the second epitaxial layer L2 is about 40% to about 100%. In thedepicted embodiment, the third epitaxial layer L3 is the top epitaxiallayer and includes SiAs, wherein, for example, a molar ratio of As inthe third epitaxial layer L3 is about 2% to about 10%. In someembodiments, a doping concentration of As in L3 is about 1×10²² to about1×10²³ atoms/cm³ (a molar ratio of As is about 2% to about 10%). In someembodiments, the epitaxial growth process may be a LPCVD process with asilicon-based precursor, a selective epitaxial growth (SEG) process, ora cyclic deposition and etching (CDE) process. For example, siliconcrystal may be grown with LPCVD with dichlorosilane (SiH₂Cl₂) as theprecursor. For another example, silicon germanium crystal may be formedwith a CDE process using HCl as the etching gas and a gas mixture ofGeH₄ and H₂ as the deposition gas. Furthermore, epitaxial layers L1-L3may be doped in-situ (during the epitaxial growth process) or ex-situ(after the epitaxial growth process is completed) with one or morep-type dopants, such as boron or indium. Similar as epitaxial S/Dfeature 250-1, a thickness T1 of the first epitaxial layer L1 is about 1nm to about 10 nm; a thickness T2 of the second epitaxial layer L2 isabout 10 nm to about 40 nm; and a thickness T3 of the third epitaxiallayer L3 is about 0.1 nm to about 20 nm. Thus, a thickness ratio of thetop (third) epitaxial layer L3 to the bottom (first) epitaxial layer L1is about 0.01 to about 20; and a thickness ratio of the top (third)epitaxial layer L3 to the second epitaxial layer L2 is about 0.0025 toabout 2. In some embodiment, a ratio of the thickness of the top (third)epitaxial layer L3 comprising SiAs to the height of epitaxial S/Dfeatures 250-3 (T1+T2+T3) is about 1% to about 20%. As depicted in FIGS.13A and 13B, epitaxial S/D features 250-3 has a merge width W1 in theY-direction and a width W2 in the X-direction. In some embodiments, themerge width W1 is about 5 nm to about 100 nm, and the width W2 is about5 nm to about 25 nm. In some embodiments, a ratio of the thickness T3 ofthe top epitaxial layer L3 to the merge width W1 of epitaxial S/Dfeatures 250-3 is about 0.001 to about 4, and a ratio of the thicknessT3 of the top epitaxial layer L3 to a width W2 of epitaxial S/D features250-3 is about 0.004 to about 4.

Referring to FIGS. 14A and 14B, portions of top epitaxial layer L3comprising SiAs are removed by a process, for example, an anisotropicalchlorine-based plasma etching process. Second epitaxial layer L2comprising SiGe is substantially unchanged due to the relatively strongselectivity between SiAs (comprised in top epitaxial layer) and SiGe. Awavy top surface of the second epitaxial layer L2 is exposed and thensource/drain contact 280 is then formed directly over the secondepitaxial layer L2. S/D contact 280 has a wavy bottom surface matching(e.g., matingly engaged with) the wavy top surface of second epitaxiallayer L2 within the landing window (trench) formed in the top epitaxiallayer L3. As depicted in FIG. 14B, S/D contact 280 contacts both thefirst epitaxial layer L1 and the second epitaxial layer L2 in theX-direction. In the depicted embodiment, the top (third) epitaxial layerL3 surrounds a lower portion of S/D contact 280. The top (third)epitaxial layer L3 includes three surfaces: surface L3S1 contacting thelower portion of the sidewall of S/D contact 280; surface L3S2contacting a portion of the second epitaxial layer L2; and surface L3S3contacting ILD 270. As depicted in FIG. 14A, the bottom surface of S/Dcontact 280 has substantially the same wavy profile as at least aportion of the top surface of the second epitaxial layer L2. The wavycontact surface has at least two crests and a valley between and belowthe two crests. The crests and the valley of the wavy contact surfaceenlarges the contact surface between S/D contact 280 and epitaxialfeature 250-3 compare to the conventional flat contact surface betweenthe S/D contact and the epitaxial S/D feature. Therefore, the contactresistance between the S/D contact and the epitaxial S/D feature isreduced and the performance of device 200 is improved.

FIGS. 15A and 15B provide a device 200 having a four-layer epitaxial S/Dfeature 250-4 in a p-type region. Epitaxial S/D feature 250-4 comprisesa first epitaxial layer L1, a second epitaxial layer L2, a thirdepitaxial layer L3, and a fourth epitaxial layer L4. The structureprofile of epitaxial S/D feature 250-4 is very similar as epitaxial S/Dfeature 250-2 as discussed above regarding FIGS. 11A and 11B. However,the materials of epitaxial S/D feature 250-4 is different than that ofepitaxial S/D feature 250-2. In the depicted embodiment, epitaxiallayers L1-L3 all comprise SiGe, but with different molar ratios of Ge.For example, a molar ratio of Ge in the first epitaxial layer L1 is lessthan about 40%; a molar ratio of Ge in the second epitaxial layer L2 isabout 40% to about 60%; and a molar ratio of Ge in the third epitaxiallayer L3 is about 60% to about 100%. In the depicted embodiment, thefourth epitaxial layer L4 is the top epitaxial layer and includes SiAs,wherein, for example, a molar ratio of As in the fourth epitaxial layerL4 is about 2% to about 10%. In some embodiments, a doping concentrationof As in L4 is about 1×10²² to about 1×10²³ atoms/cm³. Forming processof epitaxial S/D feature 250-4 is similar as that of the epitaxial S/Dfeature 250-3. In some embodiments, a thickness T1 of the firstepitaxial layer L1 is about 1 nm to about 10 nm; a thickness T2 of thesecond epitaxial layer L2 is about 10 nm to about 40 nm; a thickness T3of the third epitaxial layer L3 is about 10 nm to about 40 nm; and athickness T4 of the fourth epitaxial layer L4 is about 0.1 nm to about20 nm. Thus, a thickness ratio of the top (fourth) epitaxial layer L4 tothe bottom (first) epitaxial layer L1 is about 0.01 to about 20; athickness ratio of the top (fourth) epitaxial layer L4 to the secondepitaxial layer L2 is about 0.0025 to about 2; and a thickness ratio ofthe top (fourth) epitaxial layer L4 to the third epitaxial layer L3 isabout 0.0025 to about 2. In some embodiment, a ratio of the thickness ofthe top (fourth) epitaxial layer L4 comprising SiAs to the height ofepitaxial S/D features 250-4 (T1+T2+T3+T4) is about 1% to about 20%. Asdepicted in FIGS. 11A and 11B, epitaxial S/D features 250-4 has a mergewidth W1 in the Y-direction and a width W2 in the X-direction. In someembodiments, the merge width W1 is about 5 nm to about 100 nm, and thewidth W2 is about 5 nm to about 25 nm. In some embodiments, a ratio ofthe thickness T4 of the top epitaxial layer L4 to the merge width W1 ofepitaxial S/D features 250-4 is about 0.001 to about 4, and a ratio ofthe thickness T4 of the top epitaxial layer L4 to a width W2 ofepitaxial S/D features 250-4 is about 0.004 to about 4.

Referring to FIGS. 16A and 16B, portions of top epitaxial layer L4comprising SiAs are removed by a process, for example, an anisotropicalchlorine-based plasma etching process. The third epitaxial layer L3comprising SiGe is substantially unchanged due to the relatively strongselectivity between SiAs (comprised in top epitaxial layer) and SiGe. Awavy top surface of the third epitaxial layer L3 is exposed andsource/drain contact 280 is then formed over the third epitaxial layerL3. S/D contact 280 has a wavy bottom surface matching (matingly engagedwith) the wavy top surface of third epitaxial layer L3 within thelanding window (trench) formed in the top epitaxial layer L4. Asdepicted in FIG. 16B, in the X-direction, S/D contact 280 contacts thethird epitaxial layer L3 which contacts both the first epitaxial layerL1 and the second epitaxial layer L2. In the depicted embodiment, thetop (fourth) epitaxial layer L4 of epitaxial feature 250-4 surrounds alower portion of S/D contact 280. The top (fourth) epitaxial layer L4includes three surfaces: surface L4S1 contacting the lower portion ofthe sidewall of S/D contact 280; surface L4S2 contacting a portion ofthe third epitaxial layer L3; and surface L4S3 contacting ILD 270. Asdepicted in FIG. 16A, the bottom surface of S/D contact 280 hassubstantially the same wavy profile as at least a portion of the topsurface of the third epitaxial layer L3. The wavy contact surface has atleast two crests and a valley between and below the two crests. Thecrests and the valley of the wavy contact surface enlarges the contactsurface between S/D contact 280 and epitaxial S/D feature 250-4 compareto the conventional flat contact surface between the S/D contact and theepitaxial S/D feature. Therefore, the contact resistance between the S/Dcontact and the epitaxial S/D feature is reduced and the performance ofdevice 200 is improved.

Although not intended to be limiting, one or more embodiments of thepresent disclosure provide many benefits to a semiconductor device and aformation process thereof. For example, embodiments of the presentdisclosure form semiconductor device comprising an epitaxial S/D featurewith a top epitaxial layer of SiAs or other pentavalent silicon compoundwhich has a relatively strong etching selectivity from lower epitaxiallayer(s) (for example, comprising Si, SiC, SiP, or SiGe) of theepitaxial S/D feature. Relatively strong etching selectivity between thetop epitaxial layer and the lower epitaxial layer(s) can protect thelower epitaxial layer(s) from being damaged in the S/D contact etchingprocess. In addition, the relatively strong etching selectivity betweenthe top epitaxial layer and the lower epitaxial layer(s) can achieve amore efficient and/or more complete etching of the top epitaxial layer,so that the source/drain contact can have a wavy contact profile withthe lower epitaxial layers of the epitaxial S/D feature. The wavycontact profile results in a larger contact surface between thesource/drain contact and the epitaxial S/D feature, thus the contactresistance is reduced, and the performance of the semiconductor deviceis improved.

The present disclosure provides for many different implementations.Semiconductor device having a wavy contact surface between the S/Dfeature and the S/D contact and methods of fabrication thereof aredisclosed herein. An exemplary integrated circuit device includes a findisposed over a substrate, wherein the fin includes a channel region anda source/drain region; a gate structure disposed over the substrate andover the channel region of the fin; a source/drain feature epitaxiallygrown in the source/drain region of the fin, wherein the source/drainfeature includes a top epitaxial layer and a lower epitaxial layerformed below the top epitaxial layer, and the lower epitaxial layerincludes a wavy top surface; and a contact having a wavy bottom surfacematingly engaged with the wavy top surface of the lower epitaxial layerof the source/drain feature.

In some implementations, the top epitaxial layer of the source/drainfeature includes a pentavalent silicon compound and the lower epitaxiallayer of the source/drain feature includes a material different than thepentavalent silicon compound of the top epitaxial layer. In some furtherimplementations, the pentavalent silicon compound of the top epitaxiallayer includes silicon arsenide (SiAs). In some further implementations,a molar ratio of arsenic (As) in the SiAs is about 2% to about 10%.

In some implementations, the top epitaxial layer includes a firstsurface contacting a sidewall of the contact, a second surfacecontacting the lower epitaxial layer, and a third surface contacting aninterlayer dielectric (ILD) layer. In some further implementations, thewavy top surface of the lower epitaxial layer includes crests and avalley between and below the crests, the crests and the valley arewithin a trench formed by the first surface of the top epitaxial layer.

In some implementations, a ratio of a thickness of the top epitaxiallayer to a height of the source/drain feature is about 1% to about 20%.In some implementations, a ratio of a thickness of the top epitaxiallayer to a thickness of the lower epitaxial layer is about 0.0025 toabout 2.

Another integrated circuit device includes at least two fins disposedover a substrate, wherein each of the at least two fins include achannel region and a source/drain region; a gate structure disposed overthe substrate and over the channel regions of the at least two fins; asource/drain feature epitaxially grown in the source/drain regions ofthe at least two fins, wherein the source/drain feature includes a topepitaxial layer and a lower epitaxial layer formed below the topepitaxial layer, the lower epitaxial layer extending to wrap the atleast two fins, the top epitaxial layer including a pentavalent siliconcompound different than a material of the lower epitaxial layer; and acontact directly formed over a top surface of the lower epitaxial layerof the source/drain feature.

In some implementations, the pentavalent silicon compound of the topepitaxial layer includes silicon arsenide (SiAs) and a material of thelower epitaxial layer include silicon phosphide (SiP) or silicongermanium (SiGe). In some implementations, the top surface of the lowerepitaxial layer is a wavy top surface including crests and a valleybetween and below the crests, and the contact has a wavy bottom surfacematingly engaged with the wavy top surface of the lower epitaxial layer.In some implementations, the top epitaxial layer surrounds a lowerportion of a sidewall of the contact. In some implementations, thesource/drain feature further includes bottom epitaxial layers disposedbelow the lower epitaxial layer and wrap each of the at least two finsrespectively, a ratio of a thickness of the top epitaxial layer to athickness of the bottom epitaxial layer is about 0.01 to about 20.

An exemplary method includes forming a plurality of fins over asubstrate, wherein each of the plurality of fins comprises a channelregion and a source/drain region; forming a gate structure over thesubstrate and over the channel regions of the plurality of fins;epitaxially forming a source/drain feature including a top epitaxiallayer and a lower epitaxial layer over the source/drain regions of atleast two adjacent fins, wherein the top and lower epitaxial layers havedifferent etching selectivities; etching a portion of the top epitaxiallayer of the source/drain feature to form a trench to expose a wavy topsurface of the lower epitaxial layer of the source/drain feature;forming a contact over the wavy top surface of the lower epitaxial layerwithin the trench.

In some implementations, epitaxially forming a source/drain featurecomprises epitaxially forming a lower feature over the source/drainregion of each of the at least two adjacent fins, wherein the lowerfeatures of the at least two adjacent fins merged together to form thelower epitaxial layer having a wavy top surface; and epitaxially formingthe top epitaxial layer over the wavy top surface of the lower epitaxiallayer. In some further implementations, forming the top epitaxial layercomprises forming the top epitaxial layer to a thickness of about 0.1 toabout 20 nanometers.

In some implementations, the top epitaxial layer includes a pentavalentsilicon compound.

In some implementations, epitaxially forming a source/drain featurecomprises forming a bottom feature over the source/drain region of eachof the at least two adjacent fins before forming the lower epitaxiallayer, wherein the bottom features of the at least two adjacent finsmerged together to form a bottom epitaxial layer; forming the lowerepitaxial layer over the bottom epitaxial layer, wherein the lowerepitaxial layer having a wavy top surface; and forming the top epitaxiallayer over the wavy top surface of the lower epitaxial layer.

In some implementations, removing a portion of the top epitaxial layercomprises a chlorine(Cl)-based plasma etching process to substantiallycompletely remove a portion of the top epitaxial layer between crestsand above a valley of the wavy surface of the lower epitaxial layer. Insome further implementations, the Cl-based plasma etching processincludes removing the portion of the top epitaxial layer using a gasmixture of chlorine and boron trichloride with a power density of about0.1 W/cm2 to about 1 W/cm2 and at a pressure of about 10 to about 20mTorr.

The foregoing outlines features of several implementations so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the implementations introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A device comprising: a fin structure disposed ona substrate; a source/drain feature disposed on the fin structure, thesource/drain feature including: a first semiconductor layer disposed onthe fin structure; a second semiconductor layer disposed on the firstsemiconductor layer, the second semiconductor layer formed of adifferent material than the first semiconductor layer, the secondsemiconductor layer having a wavy top surface; a third semiconductorlayer disposed on the second semiconductor layer, the thirdsemiconductor layer including a pentavalent silicon compound; and acontact feature extending through the third semiconductor layer to thewavy top surface of the second semiconductor layer such that the contactfeature physically contacts the wavy top surface of the secondsemiconductor layer and the third semiconductor layer.
 2. The device ofclaim 1, wherein the first semiconductor layer has a first concentrationof phosphorous and the second semiconductor layer has a secondconcentration of phosphorous that is different than the firstconcentration.
 3. The device of claim 1, wherein the first semiconductorlayer has a first concentration of germanium and the secondsemiconductor layer has a second concentration of germanium that isdifferent than the first concentration.
 4. The device of claim 1,wherein the pentavalent silicon compound includes arsenic.
 5. The deviceof claim 1, wherein the pentavalent silicon compound includes a materialselected from the group consisting of antimony, bismuth and moscovium.6. The device of claim 1, further comprising a fourth semiconductorlayer disposed between the first semiconductor layer and the secondsemiconductor layer, the fourth semiconductor layer formed of adifferent material than the first semiconductor layer and the secondsemiconductor layer.
 7. The device of claim 6, wherein each of thefirst, second and third semiconductor layers includes silicon.
 8. Adevice comprising: a first fin structure disposed on a substrate; asource/drain feature disposed on the first fin structure, thesource/drain feature including: a first semiconductor layer disposed onthe first fin structure, the first semiconductor layer having a wavy topsurface; and a pentavalent silicon compound layer disposed directly onthe wavy top surface of the first semiconductor layer; and a contactfeature extending through the pentavalent silicon compound layer to thewavy top surface of the first semiconductor layer such that the contactfeature directly contacts the wavy top surface of the firstsemiconductor layer and the pentavalent silicon compound layer.
 9. Thedevice of claim 8, wherein the contact feature includes a silicidelayer.
 10. The device of claim 8, wherein the first semiconductor layerinclude a material selected from the group consisting of phosphorous andgermanium.
 11. The device of claim 8, wherein the source/drain featurefurther includes: a second semiconductor layer disposed directly on thefirst fin structure; and a third semiconductor layer disposed directlyon the second semiconductor layer, and wherein the first, second andthird semiconductor layers are each formed of different materials fromeach other.
 12. The device of claim 11, wherein the first semiconductorlayer has a first germanium concertation of about 60% to about 100%,wherein the second semiconductor layer has a second germaniumconcertation less than about 40%, and wherein the third semiconductorlayer has a third germanium concertation of about 40% to about 60%. 13.The device of claim 11, wherein the first semiconductor layer has afirst phosphorous concertation of more than about 10%, wherein thesecond semiconductor layer has a second phosphorous concertation of lessthan about 2%, and wherein the third semiconductor layer has a thirdphosphorous concertation of about 2% to about 10%.
 14. The device ofclaim 8, further comprising a second fin structure disposed on thesubstrate, and wherein a the wavy top surface of the first semiconductorlayer extends from over the first fin structure to over the second finstructure.
 15. The device of claim 8, wherein the wavy top surface ofthe first semiconductor layer include crests and a valley between andbelow the crests, and wherein the contact has a wavy bottom surfaceengaged with the wavy top surface of the first semiconductor layer. 16.A device comprising: a first fin structure and a second fin structuredisposed on a substrate; a source/drain feature disposed over the firstand second fin structures, the source/drain feature including: a firstsemiconductor layer disposed directly on the first fin structure and thesecond fin structure; a second semiconductor layer disposed directly onthe first semiconductor layer, the second semiconductor layer formed ofa different material than the first semiconductor layer, the secondsemiconductor layer having a wavy top surface; a pentavalent siliconcompound layer disposed directly on the second semiconductor layer; anda contact feature extending through the pentavalent silicon compoundlayer to the wavy top surface of the second semiconductor layer and thefirst semiconductor layer such that the contact feature physicallycontacts the pentavalent silicon compound layer, the wavy top surface ofthe second semiconductor layer and the first semiconductor layer. 17.The device of claim 16, wherein the first semiconductor layer includesSiC, and wherein the second semiconductor layer includes SiP.
 18. Thedevice of claim 16, wherein the pentavalent silicon compound layerincludes SiAs having an arsenic concertation of about 2% to about 10%.19. The device of claim 16, wherein the wavy top surface of the secondsemiconductor layer extends from at least directly over the first finstructure to at least directly over the second fin structure.
 20. Thedevice of claim 16, wherein the contact feature includes a silicidelayer and a conductive fill material.